This invention relates to generators and methods for providing increased generator output, and more particularly toward brushless generators having air-core coils in place of iron-core windings in regions of the generator that are exposed to high frequency magnetic fields.
The use of brushless generators is well known throughout various industries. For example, many automobile manufacturers utilize brushless generators to provide electrical power to vehicles. Prior brush-type generators suffered from a variety of problems related to the brushes (e.g., frequent maintenance and reduced lifespan due to the physical wearing of the brushes as they contacted the commutator bars).
In brushless generators, an exciter winding is fed a small input signal which induces a much larger signal in a rotating member. The input signal, which may be a DC current or a low frequency AC current, causes an AC current to be induced in the rotating member. The AC current is converted to DC by a rectifier assembly which is typically located within the rotating member. One example of a known rotating rectifier is described in Pinchott U.S. Pat. No. 5,065,484. The rectified DC current flows through the main windings (on the rotating member) and creates a large rotating magnetic field. The rotating field interacts with the main armature to generate a large AC signal in the armature windings. This large AC signal, which is delivered to the external load, may be effectively 10,000 times greater than signal that was input to the exciter.
In some instances, the exciter may itself be excited by a permanent magnet generator (PMG). One known example of an alternator which utilizes PMGs is described in Farr U.S. Pat. No. 4,654,551. Farr's magnetic flux field is generated by a rotating permanent ring and a toroidal control coil, where the toroidal control coil is mounted to add or subtract in the magnetic relationship with the ring.
As with most known electromagnetic devices, known brushless generators are typically manufactured using iron cores in both the exciter and main armatures. For example, Giuffrida U.S. Pat. No. 4,647,806 describes a brushless alternator having an exciter armature formed from a laminated stack of steel plates. Because the armatures are formed from laminated stacks of steel or iron plates, the armature cores tend to resemble a solid drum of iron. In view of the fact that one armature core is typically a rotating component in a brushless generator, any additional weight due to the materials used may cause premature degradation of the rotating member's bearings.
The potential for increased friction and wear on the bearings becomes even greater as the rotational speed of the generator increases. A desired increase in rotational speed is a growing trend in industry due to the fact that the output of the rotating machine tends to increase as rotational speed increases. In order to achieve greater rotational speeds, engineers must design brushless generators that are smaller and more compact than traditional designs (to simplify and reduce the effects of the higher speeds).
One known method for addressing bearing wear is to replace the conventional bearings with magnetic bearings. Unfortunately, the magnetic instability of the iron core armature would compete with the stabilizing magnetic forces of the magnetic bearing. As such, the magnetic bearing would have to be large to overcome the generator's magnetic forces.
Additional deficiencies have also been encountered in attempting to design higher speed rotational machines. One of these deficiencies is related to the use of iron cores in the armatures. As the rotational speed increases, core losses due to hysteresis and eddy-currents (collectively referred to as simply "core losses") tend to reduce the efficiency of the machines. For example, hysteresis losses tend to increase linearly with respect to frequency, while eddy-current losses tend to increase with the square of frequency. As the frequency of the machine exceeds about 24,000 rpm, the core losses may begin to offset the benefits of increases in frequency. In fact, at frequencies nearing 50,000 rpm, the core losses may be intolerable, causing the device to essentially fail.
In view of the foregoing, it is an object of this invention to provide improved brushless generators which operate at high frequencies and reduced core losses.
It is also an object of the present invention to provide improved brushless generators having reduced forces on the rotational member to increase device life.
It is also a further object of this invention to provide methods for reducing the effects of core losses on high speed rotating machines.
It is a still further object of the present invention to provide improved brushless generators having high power density in comparison to tradition iron core generators.
A single-stage brushless generator having some of the features set forth in the preamble portion of present claim 1 is known from document DE-C-954 978.